The present invention relates to a sheet feeder provided in an image forming device such as a copy machine, laser printer, and facsimile machine, and more particularly, to a sheet feeder for feeding stacked sheets of a recording medium one sheet at a time.
An example of a conventional sheet feeding device provided in image forming devices such as copy machines, laser printers, and facsimile devices is shown in FIG. 10. This sheet feeding device is provided with a feed roller 1 and a separating pad 3 opposite the feed roller 1 for conveying a sheet 2 of a recording medium one sheet at a time from a stack of sheets. The separating pad 3 is inset into a depression formed in a sheet receiving portion 5 of a separating pad holder 4. The separating pad holder 4 is urged toward the feed roller 1 by a spring 7 interposed between a fixed stop member 6 and the bottom surface of the sheet receiving portion 5. The feed roller 1 is formed of an elastic material in an approximate D-shape and includes a circumferential surface portion 8 for contacting and conveying the sheet 2 and a cutout surface portion 9 formed by cutting away a portion of the circumferential surface portion 8. Rollers 10 are rotatably mounted on both ends of the feed roller 1. Both of the rollers 10 are formed with a radius slightly smaller than the radius of the circumferential surface portion 8. Hence, when the circumferential surface portion 8 is conveying the sheet 2, the rollers 10 do not contact the sheet 2. However, when the circumferential surface portion 8 has completed conveying the sheet 2 and the cutout surface portion 9 opposes the sheet 2, the peripheral surfaces of the two rollers 10 contact the sheet 2 and oppose the urging force of the separating pad 3, thereby forming a space between the cutout surface portion 9 and the sheet 2 to prevent the cutout surface portion 9 from contacting the sheet 2.
Guide rollers 12 and 13 are positioned downstream of the feed roller 1 and convey the sheet 2 after the circumferential surface portion 8 has completed conveying the sheet 2. Although not shown in the drawings, each time the feed roller 1 conveys a sheet 2, the rotation of the feed roller 1 is suspended for a predetermined period when the cutout surface portion 9 is brought into confrontation with the sheet 2 and the two rollers 10 separate the cutout surface portion from the sheet. In this case, the rollers 10 contact and rotate on the top surface of the sheet 2, while the guide rollers 12 and 13 convey the sheet 2, thereby allowing only one sheet 2 to be conveyed at a time.
However, when conveying the sheet 2 in this way, the circumferential surface portion 8 and the rollers 10 contact the sheet 2 and separating pad 3 at different heights, because the radius of the two rollers 10 is smaller than the radius of the circumferential surface portion 8. This difference in radii creates a bump when the sheet 2 and separating pad 3 change from contacting the rollers 10 to contacting the circumferential surface 8 and vice versa, causing the separating pad 3 to move upward and downward.